1. Field of the Invention
The present invention relates to a lithium (Li) secondary cell, and more particularly, to a lithium (Li) composite oxide, a preparation method thereof, and a Li ion secondary cell adopting the Li composite oxide as an active material of a positive electrode.
2. Description of the Related Art
As small, light, wireless electronic devices, such as camcoders, cellular phones and notebook computers, have been introduced into the market, a small, light secondary cell having high energy density has been required as a power source for these devices. In this aspect, a lithium secondary cell has been spotlighted.
Lithium can be used as a negative electrode material of a battery because its electronegativity is large and it has the largest electric capacity per unit weight due to its low molecular weight. However, lithium in the metal state tends to undergo dendrite crystal growth while being passivated by the reaction with an organic solvent, causing a short within the cell. Thus, there is a problem of stability of a cell. Thus, as a negative electrode material capable of replacing lithium metal, carbonaceous material, which has the most similar electric potential to lithium, and allows a reversible intercalation/deintercalation of the lithium ions due to its layered structure, has been developed.
Electrode reaction in the lithium secondary cell constituted of a carbonaceous negative electrode, a metal oxide positive electrode and a liquid electrolyte is as follows.
During the charging period, lithium ions of the positive electrode are deintercalated, and the lithium ions of the electrolyte solution are intercalated into the layered structure of the carbonaceous material, so that the concentration of lithium ions within the electrolyte solution is constantly maintained. During the discharging period, intercalation/deintercalation of the lithium ions is performed in the reverse direction to the charging period. The cell is called a "rocking chair" battery because the lithium ions reciprocate between two electrodes during the charging/discharging periods. Also, lithium exists as ions, without participation of the lithium metal itself, thus the cell is called a "lithium ion cell"
In the above lithium ion secondary cell, the Li metal oxide is used as the positive electrode material. Particularly, LiCoO.sub.2, LuMn.sub.2 O.sub.4, LiNiO.sub.2, etc. have been used as the positive electrode material. Even though a lithium oxide containing cobalt has been commercialized, cobalt is deleterious and expensive. On the contrary, the lithium oxide containing nickel provides a high capacity and less deleteriousness at low costs. However, it is difficult to synthesize lithium oxide in a power form, and the lift span is not so good. To solve the above problems, a lithium composite oxide expressed by LiMM'O.sub.x (here, M and M' are transition metals, independently selected from cobalt (Co), manganese (Mn), nickel (Ni), vanadium (V), iron (Fe) and tungsten (W). Particularly, Ni of LiNiO.sub.2 is partially substituted with another metal, so that the synthesis becomes easy and the life span is improved. Generally, such improvement is achieved by the crystalline structure or the particle shape of the lithium oxide. For example, in the case where the particle shape of the positive electrode active material is irregular in shape and small (approximately 5 .mu.m of average diameter), a cell having high capacity can be obtained due to the smooth intercalation/deintercalcation of the lithium ions. Also, when the particle shape of the positive electrode active material is close to a spherical shape, which is advantageous to increase tap density, thus the relative weight ratio of the positive electrode active material can be increased in the preparation of the positive electrode active material.
Japanese Patent laid-open Publication No. Heisei 7-37576 discloses a method of preparing lithium oxide containing nickel. According to the disclosure, plate type monocrystalline particles (primary particles) of nickel hydroxide, educed by the neutralization reaction between a Ni salt solution and an alkaline solution, are agglomerated to form spherical or elliptic secondary particles. Then, the secondary particles of the nickel hydroxide and lithium compound, and magnesium compound if required, are thermally treated under an oxygen atmosphere, thereby forming a lithium compound containing nickel. In the case of the lithium oxide containing nickel, obtained by the above method, the flat type primary structure of the lithium oxide, where a plurality of flakes are stacked to form a layered structure, agglomerate to form the spherical or elliptical secondary structure having approximately 2.about.20 .mu.m average diameter. That is, the secondary structure of the particles is obtained through the assembling process to agglomerate the primary particles having a layered structure. However, since the secondary particles are formed by agglomerating the primary particles through the assembly process, it is difficult to make the active material fine, and special attention is required to optimize the assembling conditions.
Japanese Patent laid-open Publication No. Heisei 8-339806 discloses a lithium composite oxide containing nickel represented by LiNi.sub.(1-x) M.sub.x O.sub.2 (here, M is Co or Al, and x=0.05.about.0.3). According to this disclosure, an alkaline solution is added to a mixed aqueous solution containing cobalt salt and nickel salt to coprecipitate cobalt hydroxide and nickel hydroxide, resulting in a composite hydroxide. By agglomerating the monocrystalline particles of the composite hydroxide into a spherical or elliptic shape, a secondary structure of the particles is formed, and then lithium compound is added to the composite hydroxide and then thermally treated to form a lithium composite oxide containing cobalt in which the layered structure of the particle is exposed toward the outside of the spherical or elliptic secondary structure of the particles. According to this method, spherical particles having the secondary structure are formed at the point in time when the Ni and Co composite hydroxide is formed. Then, a lithium compound is added and then a thermal treatment is performed while the shape of the particle is maintained without changes. However, while the capacity is improved by the thermal treatment at approximately 750.degree. C., it is difficult to maintain the shape of the particles as the treatment temperature increases. Also, because the layered structure of the particle is exposed toward the outside of the secondary structure of the particle, the tap density is not high.